Is there a build-in/add-on for Oxygen to create schematron docs? (i.e. an overview of which rules are implemented?).
Currently I'm doing it by writing text in <p/> and transforming it to simple HTML by XSLT, but maybe there are better solutions?

Sorry for the late response.
In Oxygen we do not have a build-in or add-on action that generated the Schematron documentation. Also, we didn't have any requests of such an action until now. But I think it is an interesting idea. Maybe you can provide the XSLT that you have created to generate the Schematron documentation so that others users can use it and mabe improve it. You can create a GitHub project. I will add an issue on our issue tracker to provide such an action in a future version.